1. Field of the Invention
This invention relates to an excavator for laying pipelines underground, and more particularly to a pebble-stone crushing device for use in excavator for laying pipelines underground devised to crush efficiently pebble-stone or conglomerate contained in excavated earth and sand.
2. Description of the Prior Art
In the conventional excavators for laying pipelines such as, for example, a cesspipeline underground, if relatively big pebbles are contained in the earth and sand excavated by the cutter head mounted on the leading end of the excavator, there is a risk that the pebbles prevent backward smooth transport of the earth and sand in the excavator body and the sand and earth transport pipeline connected, in turn, to the rear of the cutter head. To eliminate this risk, a pebble-stone crushing device is provided which cooperates with the rear portion of the cutter head and the front plate of the excavator body to crush pebbles contained in the excavated earth and sand to conveyable sizes. The plum-pudding stone crushing devices which have so far been employed comprise a plurality of rotor teeth formed integrally with the rear face of a cutter head and which is rotatable with the cutter head, and a plurality of stator teeth which extend radially and fixed integrally to a front face plate of the excavator body, the arrangement being made such that pebbles can be crushed between these teeth. In the pebble-stone crushing device of such a construction, the innermost peripheral distance spaced between every adjacent two stator teeth is determined by the size of pebbles which can be smoothly conveyed rearwards. Accordingly, the outermost peripheral distance spaced between every adjacent two stator teeth, that is, the size of pebbles which can be crushed between these teeth is exclusively determined if the outer diameter of the excavator body is constant. Therefore, small bore excavators for laying piperlines underground comprising an excavator body with a small outside diameter are disadvantageous in that the size of pebbles which can be crushed is extremely limited.
Further, even in cases where the outside diameter of the excavator body is sufficiently large, a considerable time is necessary to crush the largest pebbles (the size of which is equivalent to the outermost peripheral distance spaced between every two adjacent stator teeth) to a conveyable size.
If, in an excavator, the quantity of pebbles which can be crushed to a conveyable size per unit time is represented by Q.sub.0, the following formula is given. EQU Q.sub.0 =(Constant/t)
where t is time for crushing.
Therefore, there is a disadvantage that if a long time is taken for crushing, Q.sub.0 ; that is, the quantity of pebbles which can be crushed per unit time is reduced so that non-crushed pebbles tend to accumulate in between the stator teeth thereby increasing the torque required to drive the cutter head or rendering the excavation and propulsion impossible in the worst case.